Detection of the CCP radical (X ²π_r) in IRC +10216: A new interstellar phosphorus-containing spectes

The CCP radical (X ² π_r) has been detected in the circumstellar gas of IRC +10216, the fifth phosphorus-bearing molecule identified in interstellar space. This identification was made on the basis of new laboratory millimeter/submillimeter direct absorption measurements, conducted in the range 120-413 GHz. Four rotational transitions of this species were observed using the Arizona Radio Observatory (ARO) 12 m telescope on Kitt Peak at 2 and 3 mm in wavelength. Each transition consists of lambda-doublets, which are well-separated in frequency in IRC +10216; five of these eight possible lines of CCP were clearly detected, while the remaining three were contaminated by stronger emission from other species. The column density derived for CCP was N_tot = 1.2 × 10¹² cm^-2 and T_rot = 21 K. Modeling of the line profiles suggests that CCP arises from an extended shell with a maximum radius of ∼40". The abundance of this radical, relative to H₂, is f ∼ 1 × 10^-9 - roughly comparable to that of PN and CP in this source. CCP may be produced from radical-radical reactions of CP, or ion-molecule chemistry involving P ⁺ and HCCH. The identification of CCP is additional evidence that phosphorus chemistrv is active in carbon-rich circumstellar gas.